1. Field of the Invention
The present invention relates to a highly durable cathode with a low hydrogen overvoltage. More particularly, it is concerned with a cathode with a low hydrogen overvoltage, which shows a very low deterioration in its properties even under an oxidizing atmosphere, and with a method for its production.
2. Discussion of the Background
There have been proposed various cathodes with a low hydrogen overvoltage, in particular, cathodes for electrolysis of an alkali metal halide aqueous solution. Of these electrodes, the one which has previously been proposed by the present applicant and is disclosed in Unexamined Japanese Patent Publication No. 112785/1979 shows remarkable effects for low hydrogen overvoltage and durability as compared with electrodes which have been known before. However, as a result of further studies, the present inventors have found that, depending on circumstances, even the electrode as disclosed in the above Unexamined Japanese Patent Publication does not always exhibit sufficient durability, and, after strenuous efforts having been made for the solution of this problem, they have accomplished the present invention.
As an industrial method of manufacturing chlorine and caustic alkali, it is already well known to obtain halogen gas from an anode compartment and an aqueous solution of caustic alkali and hydrogen gas from a cathode compartment by electrolysis in an electrolytic cell of an alkali metal halide aqueous solution. As the cathode for this electrolytic cell, a cathode with a low hydrogen overvoltage as mentioned above is used preferably. However, such an electrolytic cell is obliged to have its operation stopped in the course of its running for various reasons, and, in such case, an increase of the hydrogen overvoltage has been observed when its operation is resumed. As the result of studying this phenomenon in depth, the present inventors have discovered that in the case where the operation of the electrolytic cell is stopped by a method of short-circuiting the anode and the cathode through a bus bar, the cathode is oxidized by reverse current generated at the time of the short-circuiting, and that in the case of cathode containing nickel and cobalt as its active components, these substances become modified to hydroxides, whereby the electrode activity will decrease and will not return to the original active state even after its operation has been resumed (i.e. the hydrogen overvoltage will increase).
Moreover, it has been found that even in the case where the operation is stopped simply by ceasing conduction of electric current without short-circuiting the anode and the cathode, if the cathode is immersed over a long period of time in an aqueous solution of NaOH at a high temperature and at a high concentration, the active component of the cathode, if made of nickel or cobalt, will have a corrosion potential and will be modified into its hydroxide (this reaction is also a sort of electrochemical oxidation reaction), whereby the electrode activity decreases.